COVID-19 may be better understood if people think of it as two diseases that are distinct but linked, according to leading virologist Tony Cunningham, who heads a virus research centre at The Westmead Institute.
Professor Cunningham said mild COVID-19 was restricted to the nose and could be asymptomatic or produce symptoms similar to the common cold.
Serious COVID-19, however, is in the lungs and causes pneumonia. When this is moderate, the immune system copes with it and the patient recovers, as did US President Donald Trump. When it is severe, ventilation is needed and the person is at risk of an aberrant immune response that can be fatal.
“Before delta came along, 20- to 40-year-olds [outside the vulnerable immune suppressed group] were getting common cold symptoms and blithely spreading the virus, and older people were much more susceptible to pneumonia. With delta, younger people are getting pneumonia too,” he said.
Until COVID-19 hit the world, there were two types of coronavirus disease: one restricted to the nose and the other in the lungs.
“About 20 to 30 per cent of cases of the common cold are caused by four coronaviruses,” Professor Cunningham said.
“We can’t explain why they are so mild as we don’t know how far back they go in human civilisation. They may have been nastier originally, before human immunity controlled them and restricted them to the nose.
But in 2003, the world was hit with SARS, or severe acute respiratory syndrome, caused by a different coronavirus. It went from bats to civet cats to humans and was called a “respiratory” syndrome because it lodged in the lungs. Globally, it infected about 8000 people.
“So now we have the worst of both worlds – a virus that is as spreadable as common cold viruses and a disease that is as severe as the original SARS.”